Fix: msdms log fields emit dash instead of -1 for unset milestones

[bryancall]

Problem

The MSDMS milestone logging fields emit -1 as a literal string when a milestone is unset (i.e., difference_msec() returns -1 because one or both milestones never fired). Log parsers must treat -1 as a magic sentinel to distinguish "0 ms" (valid sub-millisecond timing) from "not applicable."

Changes

• Add unmarshal_milestone_diff() function -- outputs a single - character when the marshalled milestone difference equals the -1 sentinel (milestone was unset), instead of emitting the literal string -1. Makes log parsing cleaner: parsers distinguish "0 ms" from "not applicable" without checking for magic integers.
• Wire MSDMS container to new unmarshal function -- LogField.cc now assigns unmarshal_milestone_diff instead of the generic unmarshal_int_to_str for MSDMS fields.
• Only map the -1 sentinel, not all negatives -- preserves potentially useful debug info from reversed milestone pairs (end < start).

Testing

• Validated with milestone logging autest from Test: add msdms milestone logging field validation autest #12899
• Manual verification with traffic_logcat on binary logs
• All 15 CI platforms green

[Copilot]

Pull request overview

This PR changes how msdms (milestone-delta-in-ms) log fields are rendered in ASCII logs so that unset milestone deltas are emitted as a single - character rather than -1, making downstream parsing distinguish “not applicable” from valid numeric timings.

Changes:

• Add LogAccess::unmarshal_milestone_diff() to render milestone deltas as - when the marshalled value indicates “unset”.
• Switch LogField’s MSDMS container to use unmarshal_milestone_diff instead of the generic integer unmarshaller.
• Expose the new unmarshal helper via LogAccess.h.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/proxy/logging/LogField.cc	Uses the new milestone-delta unmarshaller for MSDMS container fields.
src/proxy/logging/LogAccess.cc	Implements unmarshal_milestone_diff() to customize formatting of milestone deltas.
include/proxy/logging/LogAccess.h	Declares the new unmarshal helper in the LogAccess API.

Comments suppressed due to low confidence (1)

src/proxy/logging/LogAccess.cc:651

• For consistency with the other integer unmarshal helpers (e.g., unmarshal_int_to_str()), the buffer-size guard here should follow the same convention (required_len < len). Using len >= 1 allows writing when the destination buffer is exactly 1 byte, where other int-to-string helpers would treat it as an overrun. Consider changing this to require 1 < len (or otherwise align it with the established checks in this file).

  if (val < 0) {
    if (len >= 1) {
      dest[0] = '-';
      return 1;
    }
    DBG_UNMARSHAL_DEST_OVERRUN
    return -1;